I just picked up a 2015 Samsung Galaxy J5 in Gold from an O2 retail store for £119.99. Currently on Amazon for £185.
There's not many left, you have to use the Click and Collect link to find a retail store that has it in stock.
Also, on the website, you can only get to it by going through Pay monthly or Browse phones link, then on the J5 screen click the "Also available on Pay As you Go" link.
The 2015 model has the same CPU / graphics and resolution as the 2016 model. Except the 2016 model has a 5.2" screen to the 2015's 5" screen.
The 2015 model also has a front facing flash, which pro' selfie takers will love.
(If you have an existing O2 number you can use it at sale in store so you don't have to buy a top up)